CBSE Class 12 Biology (2026–27)
Chapter 11: Organisms and Populations
20 Important Questions with Answers
The chapter Organisms and Populations covers population attributes, growth models, life-history variations, and population interactions such as mutualism, competition, predation, and parasitism. These are among the most important topics prescribed under the CBSE Class 12 Biology syllabus.
1. What is a population? Mention its important attributes.
Answer:
A population is a group of individuals belonging to the same species living in a particular geographical area and capable of interbreeding. Unlike an individual organism, a population possesses certain characteristics known as population attributes. These include population density, natality (birth rate), mortality (death rate), sex ratio, and age distribution. Population density refers to the number of individuals present per unit area. Natality increases population size, while mortality decreases it. Age distribution helps determine whether a population is growing, stable, or declining. These attributes are useful in understanding population dynamics and predicting future changes in the size and structure of populations.
2. Explain population density and its significance.
Answer:
Population density refers to the number of individuals of a species present in a given area or habitat at a specific time. It is an important ecological parameter because it reflects the status of a population and its interaction with the environment. Population density can be measured in terms of numbers, biomass, or percentage cover. For example, a forest may have a high density of trees but a lower density of large animals. Studying population density helps ecologists assess resource availability, competition, reproductive success, and environmental health. It also assists in wildlife conservation and management programmes by providing information about population trends.
3. Define natality and mortality. How do they influence population size?
Answer:
Natality is the rate at which new individuals are added to a population through reproduction, while mortality is the rate at which individuals die within a population. Natality increases population size, whereas mortality decreases it. Together, these two factors play a major role in determining population growth. If natality exceeds mortality, the population grows. Conversely, if mortality exceeds natality, the population declines. Environmental conditions, food availability, diseases, and predation influence both rates. Understanding natality and mortality helps ecologists predict population changes and develop strategies for species conservation and management in natural ecosystems.
4. What is age distribution? Explain its ecological importance.
Answer:
Age distribution refers to the proportion of individuals in different age groups within a population. It is usually represented by age pyramids showing pre-reproductive, reproductive, and post-reproductive individuals. A triangular pyramid indicates a growing population, a bell-shaped pyramid indicates a stable population, and an urn-shaped pyramid represents a declining population. Age distribution helps ecologists understand the reproductive potential and future growth of a population. It is useful in predicting population trends, planning conservation programmes, and assessing the long-term survival of species. Thus, age structure is a valuable indicator of population health and sustainability.
5. What are the factors responsible for changes in population size?
Answer:
Population size changes due to four major processes: natality, mortality, immigration, and emigration. Natality adds new individuals through births, while mortality removes individuals through death. Immigration refers to the movement of individuals into a population from other areas, increasing population size. Emigration is the movement of individuals out of a population, reducing its size. The combined effect of these factors determines whether a population grows, remains stable, or declines. Environmental conditions, availability of resources, and interactions with other species influence these processes. Understanding these factors helps in studying population dynamics and ecological balance.
6. Differentiate between exponential and logistic population growth.
Answer:
Exponential growth occurs when resources are unlimited and a population grows at its maximum rate. It produces a J-shaped growth curve. Under such conditions, the population increases rapidly without restrictions. Logistic growth occurs when resources become limited, causing growth to slow down after a certain point. It results in an S-shaped curve. Logistic growth considers environmental resistance and carrying capacity, which is the maximum population size that an environment can support. While exponential growth is rarely sustained in nature, logistic growth is more common because resources such as food, space, and water are generally limited in natural ecosystems.
7. What is carrying capacity?
Answer:
Carrying capacity, represented by the symbol K, is the maximum number of individuals of a species that a habitat can support sustainably with available resources. It depends on factors such as food, water, shelter, and environmental conditions. When a population reaches carrying capacity, its growth rate slows and stabilizes. Any further increase may lead to resource depletion and increased competition. Carrying capacity is an important concept in logistic population growth. It helps ecologists understand population regulation and predict how environmental changes affect species survival. Proper knowledge of carrying capacity is essential for wildlife management and conservation programmes.
8. Explain the concept of life-history variations.
Answer:
Life-history variations refer to differences in growth, reproduction, and survival strategies among organisms. Different species have evolved distinct reproductive patterns to maximize their fitness. Some organisms produce a large number of small offspring with little parental care, while others produce fewer offspring but invest more energy in their protection and development. For example, oysters release millions of eggs, whereas elephants produce only a few offspring during their lifetime. These variations help species adapt to different environmental conditions and improve survival chances. Life-history strategies are shaped by natural selection and play an important role in population dynamics and evolution.
9. What is mutualism? Give suitable examples.
Answer:
Mutualism is an interspecific interaction in which both participating species benefit from the association. It is represented by (+,+). Mutualistic relationships enhance survival, growth, or reproduction of both partners. A classic example is the association between lichens, where algae provide food through photosynthesis and fungi offer protection and moisture. Another example is the relationship between flowering plants and pollinators such as bees. The plants receive pollination, while the insects obtain nectar as food. Mutualism contributes significantly to ecosystem stability and biodiversity by promoting cooperation among species and ensuring successful reproduction and resource utilization.
10. Describe competition as a population interaction.
Answer:
Competition occurs when two or more organisms require the same limited resource such as food, water, light, or space. It is represented by (-,-) because both competitors are negatively affected. Competition may occur within the same species (intraspecific) or between different species (interspecific). Strong competition can reduce growth, reproduction, and survival rates. According to the competitive exclusion principle, two species competing for identical resources cannot coexist indefinitely; one species eventually outcompetes the other. Competition plays an important role in regulating population size and promoting adaptations that reduce resource overlap among species in natural ecosystems.
11. What is predation? Mention its ecological significance.
Answer:
Predation is an interaction in which one organism, called the predator, captures and feeds on another organism, known as the prey. It is represented by (+,-). Predators obtain food and energy, while prey populations are reduced. Predation helps regulate prey populations and prevents overexploitation of resources. It also promotes natural selection by eliminating weak or less adapted individuals. Predators contribute to maintaining species diversity and ecosystem stability. Examples include lions feeding on deer and ladybird beetles feeding on aphids. Thus, predation is an essential ecological process that helps maintain balance within biological communities.
12. Explain parasitism with examples.
Answer:
Parasitism is an interaction where one organism, called the parasite, benefits while the host is harmed. It is represented by (+,-). Parasites depend on hosts for food, shelter, or reproduction. They may live on the surface of the host (ectoparasites) or inside the body (endoparasites). Examples include lice on humans, tapeworms in the intestine, and Cuscuta growing on host plants. Parasites often evolve adaptations such as hooks, suckers, and high reproductive capacity to ensure survival. Although parasites harm hosts, they usually do not kill them immediately because their own survival depends on the host.
13. What are ectoparasites and endoparasites?
Answer:
Ectoparasites are parasites that live on the external surface of the host’s body. Examples include lice, ticks, and mosquitoes. They feed on blood or body fluids without entering the host’s internal tissues. Endoparasites live inside the host’s body, such as tapeworms, roundworms, and Plasmodium. Endoparasites are highly specialized and often possess adaptations like resistant body coverings and strong attachment organs. Both types of parasites obtain nourishment from the host and may cause diseases or weaken the host. These interactions significantly influence population health, survival, and ecological relationships within communities.
14. What is commensalism? Give examples.
Answer:
Commensalism is an interaction between two species in which one species benefits while the other is neither harmed nor benefited. It is represented by (+,0). A common example is the orchid growing on mango trees. The orchid gains support and better exposure to sunlight, while the tree remains unaffected. Another example is cattle egrets feeding on insects disturbed by grazing cattle. The birds benefit from easy access to food, whereas the cattle are neither helped nor harmed. Commensalism allows one species to utilize resources or opportunities created by another species without causing damage or competition.
15. Explain the competitive exclusion principle.
Answer:
The competitive exclusion principle was proposed by Gause. It states that two species competing for the same limiting resources cannot coexist indefinitely in the same habitat. Eventually, one species becomes more efficient in resource utilization and outcompetes the other, leading to its elimination or migration. This principle explains why species occupying similar ecological niches often show differences in resource use. To avoid exclusion, species may undergo resource partitioning, where they utilize different resources or occupy different habitats. The principle highlights the role of competition in shaping community structure and maintaining ecological balance.
16. What is resource partitioning?
Answer:
Resource partitioning is the process by which competing species use different resources or utilize the same resource in different ways to reduce competition. This allows similar species to coexist in the same habitat. For example, different species of birds may feed at different heights on the same tree. By dividing available resources, organisms minimize direct competition and improve survival chances. Resource partitioning promotes biodiversity and ecological stability by enabling multiple species to share habitats successfully. It is considered an important evolutionary adaptation that helps avoid competitive exclusion and supports long-term coexistence in ecosystems.
17. Describe adaptations of desert plants.
Answer:
Desert plants possess several adaptations that help them survive under conditions of extreme heat and water scarcity. Many have thick, waxy cuticles that reduce water loss through transpiration. Leaves are often reduced to spines, as seen in cacti, minimizing surface area and water loss. Stems become green and photosynthetic. Deep or extensive root systems help absorb available water efficiently. Some plants use CAM photosynthesis, opening stomata at night to reduce evaporation. These adaptations enable desert plants to conserve water, withstand harsh conditions, and maintain growth and reproduction in arid environments.
18. Explain adaptations found in kangaroo rats.
Answer:
Kangaroo rats are desert rodents that exhibit remarkable physiological adaptations for water conservation. They rarely drink water directly and obtain moisture from the metabolic breakdown of food. Their kidneys are highly efficient and produce very concentrated urine, minimizing water loss. They remain in burrows during the daytime to avoid excessive heat and emerge mainly at night. Their feces are dry, further reducing water wastage. These adaptations help kangaroo rats survive in extremely dry environments where water is scarce. Such examples demonstrate how natural selection shapes organisms to fit challenging environmental conditions.
19. Why are predators considered beneficial in ecosystems?
Answer:
Predators are beneficial because they regulate prey populations and prevent overpopulation. By removing weak, diseased, or old individuals, predators help maintain healthy prey populations. They also prevent any one species from dominating an ecosystem, thereby promoting biodiversity. Predators play an important role in energy transfer across trophic levels and contribute to ecological balance. In agriculture, biological control agents such as ladybird beetles help manage pest populations naturally, reducing the need for chemical pesticides. Thus, predators are essential for maintaining stability, diversity, and proper functioning of ecosystems.
20. Compare mutualism and parasitism.
Answer:
Mutualism and parasitism are two important interspecific interactions. In mutualism, both participating species benefit from the relationship, represented by (+,+). Examples include lichens and plant-pollinator interactions. Mutualism enhances survival and reproductive success of both partners. In parasitism, only the parasite benefits while the host is harmed, represented by (+,-). Examples include tapeworms in humans and Cuscuta on host plants. Parasites obtain food and shelter from hosts and may cause disease or reduced fitness. While mutualism promotes cooperation and ecosystem stability, parasitism involves exploitation of one organism by another. Both interactions influence population dynamics and ecological relationships.